Dual-Functional Chemical Pre-Sodiation of Carbon-Coated Hard Carbon Anodes with Initial Coulombic Efficiency up to 99.5% for Sodium-Ion Batteries
Hard carbon (HC) is a promising anode material for sodium-ion batteries (SIBs) but suffers from low initial Coulombic efficiency (ICE) and unstable solid electrolyte interphase (SEI). Here, we report a dual-functional strategy combining surface engineering and solution chemical pre-sodiation. A graphitic carbon coating on HC acts as a conductive buffer network and shields surface defects, while sodium biphenyl (Na-Bp) pre-sodiation drives sodium ions into the material via a potential difference, inducing a pre-SEI layer that matures into a thin, dense, NaF-rich inorganic SEI during cycling. This approach compensates for irreversible sodium loss and enhances cycling stability. The pre-sodiated electrode (pCH4-HC) achieves an ICE of 99.5% and a reversible capacity of 321.7 mAh g−1, compared to 54.2% for untreated HC. Long-term cycling shows 74.0% capacity retention after 1000 cycles at 300 mA g−1. In full-cells with NaNi1/3Fe1/3Mn1/3O2 (NFM) cathode, pCH4-HC||NFM delivers 81.9 mAh g−1 after 100 cycles, demonstrating excellent stability and rate performance. This dual-strategy approach validates the adaptability of pre-sodiation technology for high-performance SIBs.